Flame-retardant polyamide compositions are well known in the art. The usual way of making polyamide flame-resistant is by the addition of halogen compounds together with a synergist, usually antimony trioxide. The disadvantage of this method is that poisonous and corrosive gases are released when the finished products become exposed to heat or an open flame. To eliminate this drawback, substitution of the known additives with low molecular weight nitrogen compounds, in particular cyanuric acid, melamine or their compounds which form melamine cyanurate. An example of this can be seen in DE-A No. 27 40 092 which describes the use of melamine cyanurate as a flameproofing agent for polyamide. The disadvantages here are that the parameters for processing are very narrow and the polymer mass deteriorates and flows very quickly under the influence of heat or an open flame.
If stiffening fibers such as glass fibers are also incorporated, the so-called "wicking effect" takes place. This means that the stiffening fibers act like the wick of a candle and actually promote burning. This class of compounds is therefore useless in cases where stiffened compositions are required.
It is recognized that known flame-retardant additives have a strong tendency towards surface migration. This adversely affects the electrical properties of bodies made therefrom. The problem is particularly acute for nitrogen and halogen compounds.
Some of the foregoing disadvantages are substantially overcome by materials such as aluminum hydroxide. This is available in finely divided, substantially spherical form, so that it can be readily introduced into elastomers without adversely effecting their flexibility. As such, these compositions find use as cable insulators where flame-resistance and flexibility are of substantial importance.
The action of aluminum hydroxide is based upon the elimination of water which starts at 190.degree. C. As a result, it is useless in thermoplastic polyamide compositions, as they must be processed at much higher temperatures. Therefore, magnesium hydroxide, which releases water beginning at 340.degree. C., has been considered as a flame-retardant for use in connection with such compositions. The flame-retardant effect is brought about by a combination of factors; namely, the cooling effect resulting from the release of water under the influence of heat, the formation of a protective gas screen which inhibits the access of oxygen, and the formation of a protective magnesium oxide layer or coating.
The preparation and use of a form of magnesium hydroxide as a flame-retardant for polypropylene has been described in "Plastics and Rubber Processing and Applications", 6 (1986) 169-175. The specific magnesium hydroxide taught is obtained from seawater in the usual manner and has a specific surface area of 40 m.sup.2 /g.
However, it has been necessary to use 50% to 60% by weight magnesium hydroxide and aluminum hydroxide in order to obtain the desired properties. In view of this high concentration, specialized mixing devices must be used in order to prepare them. In fact, JP-A-12943/1978 teaches the addition of large amounts of unsaturated metal soaps in order to obtain 70% by weight magnesium hydroxide compositions.
In U.S. Pat. No. 4,098,762, there is disclosed magnesium hydroxide which has been coated with an anionic surfactant; namely, sodium stearate. This composition is taught as being suitable as a flame-retardant additive for non-polar and strongly hydrophobic polymers, i.e. polyolefins.
A variation of the foregoing is found in EP-A-No. 52868. This reference describes a specific combination of a thermoplastic resin, magnesium hydroxide (which has been coated with an alkali oleate), and 0.1% to 10% by weight of magnesium and/or aluminum oleate. The minimum thickness of a test body obtained therefrom was 3.2 mm and, thus, the production of thin bodies is contra-indicated.
Moreover, the presence of unsaturations in the oleates makes them extremely sensitive to oxidation. As a result, they start to decompose at processing temperatures of approximately 300.degree. C. The unsaturations cause the composition to discolor and its mechanical and flame-resistant properties to deteriorate. In addition, as in all such unsaturated compositions, continued use in the presence of heat and/or light causes oxidation which, in turn, leads to yellowing and other color changes.